In 2006, my colleague Carl Schwartz of Massachusetts General Hospital, using MRI scans, began evaluating the anatomy and brain function of 135 of these children when they became 18. He found that the high-reactive children had a thicker cortex in a small area in the ventromedial cortex of the right hemisphere. This area sends impulses to the amygdala, as well as the hypothalamus, periaqueductal gray (also known as the central gray), and autonomic nervous system. Activity in these target sites explains why the older, high-reactive children continued to be vigilant to novelty and have a highly-reactive sympathetic nervous system. It also accounts for why they arched their backs so often at 4 months of age.

As these young adults lay in an MRI scanner, they were shown a series of familiar and unfamiliar faces and objects. Schwartz's preliminary, as yet unpublished, observations reveal that the high-reactive children showed greater activity to the unfamiliar stimuli than the low-reactive children in the right amygdala and right ventromedial cortex. The high-reactive 18-year-olds who showed this brain profile most clearly were the ones who'd displayed the most distress at 4 months.

I never expected to live long enough to see any neuroanatomical or neurobiological evidence to support my early hunch that the high-reactive children were born with an excitable amygdala. Schwartz's findings don't imply that infants inherit a fixed propensity for being nervous or relaxed, introverted or extroverted. They only suggest that some temperamental biases can persist, and exert an influence on the moods of some young adults. Thus, when we consider the many influences on human development, temperament must be in the mix. But a temperamental bias doesn't determine any one personality trait or symptom of mental illness, any more than the experiences of the first year of life or the legacy of a specific gene do. The power of the temperamental bias is to make it more difficult to acquire certain traits. Let me explain. The probability that a high reactive infant will become an 18-year-old who's shy, timid, and socially anxious is about 20 percent. But the probability that this infant won't grow up to be an outgoing, bold, and exuberant adult is 90 percent. Thus, the temperamental bias predicts with great accuracy what the child won't become.

Recall that this conclusion resembles the one that Mary Ainsworth arrived at in her book Infancy in Uganda. She wrote, "We must concede that there are genetically-based individual differences between babies. . . . It is quite impossible to differentiate genetic, prenatal, and perinatal influences from environmental influences." she described one infant she'd classified as "non-attached" because the girl didn't cry or attempt to follow the mother when she left the room. At the same time, this child struck Ainsworth as happy and comfortable. I suspect this girl belonged to the low-reactive temperamental group. Ainsworth also described a small group of infants classified as "insecurely attached," because they cried frequently (she'd later call this attachment Type C, insecure-resistant). But always a careful observer, she noted that two of the fussy infants were chronically malnourished—which may well have explained the constant crying. Further, she observed that many of the frequently fussy babies cried not only when their mother left their side, but also when they were held by the mother. In a telling sentence toward the end of the book, she wrote: "Therefore, the warmth of the mother and her observed affectionate contact behavior do not explain the differences between these groups." There was something else going on, and Ainsworth postulated that the "something" might be genetically based, individual differences between babies.